Methods and systems for watermark processing of line art images

ABSTRACT

Line art on banknotes and the like is subtly altered to steganographically encode plural-bit digital information. For example, the lines&#39; widths or spacings can be locally changed so as to slightly modulate the apparent luminance of the document. The encoded information can be decoded using suitably-equipped processing equipment and used to identify a document as a banknote. Various anti-counterfeiting actions&#39; can then be taken.

RELATED APPLICATION DATA

[0001] This application is a continuation of copending utilityapplication Ser. No. 09/074,034, filed May 6, 1998. The application Ser.No. 09/074,034 is a continuation-in-part of utility application Ser. No.08/438,159 (now U.S. Pat. No. 5,850,481), filed May 8, 1995, andprovisional application Ser. No. 60/082,228, filed Apr. 16, 1998.

[0002] (The specification of application Ser. No. 08/438,159 issubstantially identical to that of applicant's issued Pat. Nos.5,636,292, 5,710,834, 5,748,763, 5,748,783. The specification ofapplication Ser. No. 60/082,228 is attached hereto as Appendix A.)

[0003] The subject matter of this application is also related to that ofthe present assignee's other pending applications, namely Ser. Nos.08/508,083 (now U.S. Pat. No. 5,841,978), 08/534,005 (now U.S. Pat. No.5,832,119), 08/637,531 (now U.S. Pat. No. 5,822,436), 08/649,419 (nowU.S. Pat. No. 5,862,260), 08/746,613 (now U.S. Pat. No. 6,122,403),08/951,858 (now U.S. Pat. No. 6,026,193), and 08/969,072 (now U.S. Pat.No. 5,809,160), and the allowed applications referenced below.

FIELD OF THE INVENTION

[0004] The present invention relates to methods and systems forinconspicuously embedding binary data in line art images (such as areused in currency and the like), and associated methods/systems fordecoding such data from such images. One application of such technologyis discouraging counterfeiting of banknotes.

BACKGROUND AND SUMMARY OF THE INVENTION

[0005] Watermarking is a quickly growing field of endeavor, with severaldifferent approaches. The present assignee's work is reflected in U.S.Pat. Nos. 5,710,834, 5,636,292, 5,721,788, 5,748,763, 5,748,783, and5,745,604, in allowed U.S. application Ser. Nos. 08/327,426 (now U.S.Pat. No. 5,768,426), 08/508,083 (now U.S. Pat. No. 5,841,978),08/438,159 (now U.S. Pat. No. 5,850,481), and in laid-open PCTapplication WO97/43736. (The laid-open PCT application is substantiallyidentical to the disclosure of U.S. Pat. No. 6,122,403.) Other work isillustrated by U.S. Pat. Nos. 5,734,752, 5,646,997, 5,659,726,5,664,018, 5,671,277, 5,687,191, 5,687,236, 5,689,587, 5,568,570,5,572,247, 5,574,962, 5,579,124, 5,581,500, 5,613,004, 5,629,770,5,461,426, 5,743,631, 5,488,664, 5,530,759, 5,539,735, 4,943,973,5,337,361, 5,404,160, 5,404,377, 5,315,098, 5,319,735, 5,337,362,4,972,471, 5,161,210, 5,243,423, 5,091,966, 5,113,437, 4,939,515,5,374,976, 4,855,827, 4,876,617, 4,939,515, 4,963,998, 4,969,041, andpublished foreign applications WO 98/02864, EP 822,550, WO 97/39410, WO96/36163, GB 2,196,167, EP 777,197, EP 736,860, EP 705,025, EP 766,468,EP 782,322, WO 95/20291, WO 96/26494, WO 96/36935, WO 96/42151, WO97/22206, WO 97/26733. Some of the foregoing patents relate to visiblewatermarking techniques. Other visible watermarking techniques (e.g.data glyphs) are described in U.S. Pat. No. 5,706,364, 5,689,620,5,684,885, 5,680,223, 5,668,636, 5,640,647, 5,594,809.

[0006] Most of the work in watermarking, however, is not in the patentliterature but rather in published research. In addition to thepatentees of the foregoing patents, some of the other workers in thisfield (whose watermark-related writings can by found by an author searchin the INSPEC database) include I. Pitas, Eckhard Koch, Jian Zhao,Norishige Morimoto, Laurence Boney, Kineo Matsui, A. Z. Tirkel, FredMintzer, B. Macq, Ahmed H. Tewfik, Frederic Jordan, Naohisa Komatsu, andLawrence O'Gorman.

[0007] The artisan is assumed to be familiar with the foregoing priorart.

[0008] In the following disclosure it should be understood thatreferences to watermarking encompass not only the assignee'swatermarking technology, but can likewise be practiced with any otherwatermarking technology, such as those indicated above.

[0009] Watermarking can be applied to myriad forms of information. Thepresent disclosure focuses on its applications to line art imagery, ofthe sort typically employed in banknotes, travelers checks, passports,stock certificates, and the like (hereafter collectively referred to as“banknotes”). However, it should be recognized that the principlesdiscussed below can also be applied outside this particular field.

[0010] Most of the prior art in image watermarking has focused onpixelated imagery (e.g. bit-mapped images, JPEG/MPEG imagery, VGA/SVGAdisplay devices, etc.). In most watermarking techniques, the luminanceor color values of component pixels are slightly changed to effectsubliminal encoding of binary data through the image. (This encoding canbe done directly in the pixel domain, or in another domain, such as theDCT domain.) In some systems, isolated pixels are changed in accordancewith one or more bits of the binary data; in others, pluraldomain-related groupings of pixels (e.g. locally adjoining, orcorresponding to a given DCT component) are so changed. In all cases,however, pixels have served as the ultimate carriers of the embeddeddata.

[0011] While pixelated imagery is a relatively recent development, lineart goes back centuries. One familiar example is U.S. paper currency. Onthe one dollar banknote, for example, line art is used in severaldifferent ways. One is to form intricate webbing patterns around themargin of the note (generally comprised of light lines on darkbackground). Another is so form grayscale imagery, such as the portraitof George Washington (generally comprised of dark lines on a lightbackground).

[0012] There are two basic ways to simulate grayscales in line art. Oneis to change the relative spacings of the lines to effect a lighteningor darkening of an image region. FIG. 1A shows such an arrangement; areaB looks darker than area A due to the closer spacings of the componentlines. The other technique is to change the widths of the componentlines—wider lines resulting in darker areas and narrower lines resultingin lighter areas. FIG. 1B shows such an arrangement. Again, area B looksdarker than area A, this time due to the greater widths of the componentlines. These techniques are often used together.

[0013] In my prior applications, I noted that conventional watermarkingtechniques are unsuitable for use with a type of line art known asvector graphics. (In vector graphics, lines are digitally described byreference to their geometry.) In particular, I noted that a change ofeven a single bit in a vector graphic can have substantial, unintendedeffects (e.g. changing a circle to a square), making the subliminalencoding of binary watermark data difficult.

[0014] In those prior applications, I proposed various solutions to thisproblem. One solution was to recognize that the eye is relativelyinsensitive to the precise placement and/or contours of a line,permitting slight modulation to effect information encoding. Inparticular, I noted:

[0015] “The primary example is the borders and contours between where agiven line or figure is drawn or not drawn, or exactly where a bit-mapchanges from green to blue. In most cases, a human viewer of suchgraphics will be keenly aware of any attempts to “modulate signaturesignals” via the detailed and methodical changing of the precisecontours of a graphic object. Nevertheless, such encoding of thesignatures is indeed possible. The distinction between this approach andthat disclosed in the bulk of this disclosure is that now the signaturesmust ultimately derive from what already exists in a given graphic,rather than being purely and separately created and added into a signal.This disclosure points out the possibilities here nonetheless. The basicidea is to modulate a contour, a touch right or a touch left, a touch upor a touch down, in such a way as to communicate an N-bit identificationword. The locations of the changes contours would be contained in a ananalogous master noise image, though now the noise would be a record ofrandom spatial shifts one direction or another, perpendicular to a givencontour. Bit values of the N-bit identification word would be encoded,and read, using the same polarity checking method between the appliedchange and the change recorded in the master noise image.”

[0016] The present disclosure expands on these principles by referenceto several illustrative embodiments.

[0017] One embodiment posits a virtual grid of points imposed on a lineart image (e.g. a U.S. one dollar banknote), with the points spaced atregular intervals in vertical and horizontal directions. (The horizontaland vertical intervals need not be equal.) The virtual points may beimposed over some or all of the bill at equal vertical and horizontalspacings of 250 μm. In regions of the banknote having line art, thecomponent lines of the art snake in and amongst these virtual gridpoints.

[0018] Each grid point is considered to be the center of arounded-square region. The luminance of the region is a function of theproximity of any line(s) within the boundary of the region to theregion's centerpoint, and the thickness of the line(s).

[0019] To change the luminance of the region, the contour of the line(s)is changed slightly within the region. In particular, the line is madeslightly thicker to decrease luminance; or thinner to increaseluminance. (Unless otherwise noted, dark lines on light backgrounds arepresumed.) The ability to effect these slight changes is then employed,in accordance with known pixelation-based watermarking techniques, toencode binary data in the line art. If such a banknote is thereafterscanned by a scanner, the values of the pixel data produced by thescanner will reflect the foregoing alterations in luminance values,permitting embedded watermark data to be decoded.

[0020] In an alternative embodiment, the line widths are not changed.Instead, the positions of the lines are shifted slightly towards or awayfrom certain virtual grid points to effect an increase or decrease inthe corresponding area's luminosity, with the same effect. Otherembodiments are also detailed.

[0021] By the techniques disclosed herein, line art images can beencoded to subliminally convey binary data. This capability permitsvarious hardware systems to recognize banknotes, and to change or limittheir actions in a predetermined manner (e.g. a photocopier equippedwith this capability can refuse to reproduce banknotes, or can insertforensic tracer data in the copy).

[0022] The foregoing features and advantages of the invention will bemore readily apparent from the following detailed description, whichproceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIGS. 1A and 1B show prior art techniques for achieving grayscaleeffects using line art.

[0024]FIG. 2 shows a virtual array of grid points that can be imposed onan image according to one embodiment of the present invention.

[0025]FIG. 3 shows a virtual array of regions that can be imposed on animage according to the FIG. 2 embodiment.

[0026]FIG. 4 shows an excerpt of FIG. 3 with a line from a line artimage passing therethrough.

[0027]FIG. 5 shows changes to the width of the line of FIG. 3 to effectwatermark encoding according to one embodiment of the present invention.

[0028]FIG. 6 shows changes to the position of the line of FIG. 3 toeffect watermark encoding according to another embodiment of the presentinvention.

[0029]FIG. 7 is a block diagram of a photocopier according to anotherembodiment of the invention.

DETAILED DESCRIPTION

[0030] Referring to FIG. 2, an illustrative form of the inventionemploys a grid 10 of imaginary reference points arrayed over a line artimage. The spacing between points is 250 μm in the illustratedarrangement, but greater or lesser spacings can of course be used.

[0031] Associated with each grid point is a surrounding region 12, shownin FIG. 3. As described below, the luminosity (or reflectance) of eachof these regions 12 is slightly changed to effect the subliminalencoding of binary data.

[0032] Region 12 can take various shapes; the illustratedrounded-rectangular shape is representative only. (The illustrated shapehas the advantage of encompassing a fairly large area while introducingfewer visual artifacts than, e.g., square regions.) In otherembodiments, squares, rectangles, circles, ellipses, etc., canalternatively be employed.

[0033]FIG. 4 is a magnified view of an excerpt of FIG. 3, showing a line14 passing through the grid of points. The width of the line, of course,depends on the particular image of which it is a part. The illustratedline is about 25 μm in width; greater or lesser widths can naturally beused.

[0034] In a first embodiment of the invention, shown in FIG. 5, thewidth of the line is controllably varied so as to change the luminosityof the regions through which it passes. To increase the luminosity (orreflectance), the line is made narrower (i.e. less ink in the region).To decrease the luminosity, the line is made wider (i.e. more ink).

[0035] Whether the luminance in a given region should be increased ordecreased depends on the particular watermarking algorithm used. Anyalgorithm can be used, by changing the luminosity of regions 12 as thealgorithm would otherwise change the luminance or colors of pixels in apixelated image.

[0036] In an exemplary algorithm, the binary data is represented as asequence of −1s and 1s, instead of 0s and 1s. (The binary data cancomprise a single datum, but more typically comprises several. In anillustrative embodiment, the data comprises 100 bits.)

[0037] Each element of the binary data sequence is then multiplied by acorresponding element of a pseudo-random number sequence, comprised of−1s and 1s, to yield an intermediate data signal. Each element of thisintermediate data signal is mapped to a corresponding sub-part of theimage, such as a region 12. The image in (and optionally around) thisregion is analyzed to determine its relative capability to concealembedded'data, and a corresponding scale factor is produced. Exemplaryscale factors may range from 0 to 3. The scale factor for the region isthen multiplied by the element of the intermediate data signal mapped tothe region in order to yield a “tweak” value for the region. In theillustrated case, the resulting tweaks can range from −3 to 3. Theluminosity of the region is then adjusted in accordance with the tweakvalue. A tweak value of −3 may correspond to a −5% change in luminosity;−2 may correspond to −2% change; −1 may correspond to −1% change; 0 maycorrespond to no change; 1 may correspond to +1% change; 2 maycorrespond to +2% change, and 3 may correspond to +5% change. (Thisexample follows the basic techniques described in the Real Time Encoderembodiment disclosed in U.S. Pat. No. 5,710,834.)

[0038] In FIG. 5, the watermarking algorithm determined that theluminance of region A should be reduced by a certain percentage, whilethe luminance of regions C and D should be increased by certainpercentages.

[0039] In region A, the luminance is reduced by increasing the linewidth. In region D, the luminance is increased by reducing the linewidth; similarly in region C (but to a lesser extent).

[0040] No line passes through region B, so there is no opportunity tochange the region's luminance. This is not fatal to the method, however,since the watermarking algorithm redundantly encodes each bit of data insub-parts spaced throughout the line art image.

[0041] The changes to line widths in regions A and D of FIG. 5 areexaggerated for purposes of illustration. While the illustrated varianceis possible, most implementations will modulate the line width 3-50%(increase or decrease).

[0042] (Many watermarking algorithms routinely operate within a signalmargin of about +/−1% changes in luminosity to effect encoding. That is,the “noise” added by the encoding amounts to just 1% or so of theunderlying signal. Lines typically don't occupy the full area of aregion, so a 10% change to line width may only effect a 1% change toregion luminosity, etc. Banknotes are different from photographs in thatthe art need not convey photorealism. Thus, banknotes can be encodedwith higher energy than is used in watermarking photographs, providedthe result is still aesthetically satisfactory. To illustrate, localizedluminance changes on the order of 10% are possible in banknotes, whilesuch a level of watermark energy in photographs would generally beconsidered unacceptable. In some contexts, localized luminance changesof 20, 30, 50 or even 100% are acceptable.)

[0043] In the illustrated embodiment, the change to line width is afunction solely of the tweak to be applied to a single region. Thus, ifa line passes through any part of a region to which a tweak of 2% is tobe applied, the line width in that region is changed to effect the 2%luminance difference. In variant embodiments, the change in line widthis a function of the line's position in the region. In particular, thechange in line width is a function of the distance between the region'scenter grid point and the line's closest approach to that point. If theline passes through the grid point, the full 2% change is effected. Atsuccessively greater distances, successively less change is applied. Themanner in which the magnitude of the tweak changes as a function of lineposition within the region can be determined by applying one of variousinterpolation algorithms, such as the bi-linear, bi-cubic, cubicsplines, custom curve, etc.

[0044] In other variant embodiments, the change in line width in a givenregion is a weighted function of the tweaks for adjoining or surroundingregions. Thus, the line width in one region may be increased ordecreased in accordance with a tweak value corresponding to one or moreadjoining regions.

[0045] Combinations of the foregoing embodiments can also be employed.

[0046] In the foregoing embodiments, it is sometimes necessary totrade-off the tweak values of adjoining regions. For example, a line maypass along a border between regions, or pass through the pointequidistant from four grid points (“equidistant zones”). In such cases,the line may be subject to conflicting tweak values—one region may wantto increase the line width, while another may want to decrease the linewidth. (Or both may want to increase the line width, but differingamounts.) Similarly in cases where the line does not pass through anequidistant zone, but the change in line width is a function of aneighborhood of regions whose tweaks are of different values. Again,known interpolation functions can be employed to determine the weight tobe given the tweak from each region in determining what change is to bemade to the line width in any given region.

[0047] In the exemplary watermarking algorithm, the average change inluminosity across the image is zero, so no generalized lightening ordarkening of the image is apparent. The localized changes in luminosityare so minute in magnitude, and localized in position, that they areessentially invisible (e.g. inconspicuous/subliminal) to human viewers.

[0048] An alternative embodiment is shown in FIG. 6, in which linePosition is changed rather than line width.

[0049] In FIG. 6 the original position of the line is shown in dashedform, and the changed position of the line is shown in solid form. Todecrease a region's luminosity, the line is moved slightly closer to thecenter of the grid point; to increase a region's luminosity, the line ismoved slightly away. Thus, in region A, the line is moved towards thecenter grid point, while in region D it is moved away.

[0050] It will be noted that the line on the left edge of region A doesnot return to its nominal (dashed) position as it exits the region. Thisis because the region to the left of region A also is to have decreasedluminosity. Where possible, it is generally preferable not to return aline to its nominal position, but instead permit shifted lines to remainshifted as they enter adjoining regions. So doing permits a greater netline movement within a region, increasing the embedded signal level.

[0051] Again, the line shifts in FIG. 6 are somewhat exaggerated. Moretypical line shifts are on the order of 3-50 μm.

[0052] One way to think of the FIG. 6 embodiment is to employ amagnetism analogy. The grid point in the center of each region can bethought of as a magnet. It either attracts or repels lines. A tweakvalue of −3, for example, may correspond to a strong-valued attractionforce; a tweak value of +2 may correspond to a middle-valued repulsionforce, etc. In FIG. 6, the grid point in region A exhibits an attractionforce (i.e. a negative tweak value), and the grid point in region Dexhibits a repulsion force (e.g. a positive tweak value).

[0053] The magnetic analogy is useful because the magnetic effectexerted on a line depends on the distance between the line and the gridpoint. Thus, a line passing near a grid point is shifted more inposition than a line near the periphery of the region.

[0054] Each of the variants discussed above in connection with FIG. 5 islikewise applicable to FIG. 6.

[0055] Combinations of the embodiments of FIGS. 5 and 6 can of course beused, resulting in increased watermark energy, better signal-to-noiseratio and, in many cases, less noticeable changes.

[0056] In still a further embodiment, the luminance in each region ischanged while leaving the line unchanged. This can be effected bysprinkling tiny dots of ink in the otherwise-vacant parts of the region.In high quality printing, of the type used with banknotes, droplets onthe order of 3 μm in diameter can be deposited. (Still larger dropletsare still beyond the perception threshold for most viewers.) Speckling aregion with such droplets (either in a regular array, or random, oraccording to a desired profile such as Gaussian), can readily effect a1% or so change in luminosity. (Usually'dark droplets are added to aregion, effecting a decrease in luminosity. Increases in luminosity canbe effected by speckling with a light colored ink, or by forming lightvoids in line art otherwise present in a region.)

[0057] In a variant of the speckling technique, very thin mesh lines canbe inserted in the artwork—again to slightly change the luminance of oneor more regions.

[0058] Although not previously mentioned, it is contemplated that thebanknote will include some manner of calibration information tofacilitate registration of the image for decoding. This calibrationinformation can be steganographic or overt. Several techniques forsteganographically embedding calibration information are disclosed in myprior patents and applications. Other techniques can be found in othersof the cited work.

[0059] To decode watermark data, the encoded line art image must beconverted into electronic form for analysis. This conversion istypically performed by a scanner.

[0060] Scanners are well known, so a detailed description is notprovided here. Suffice it to say that scanners conventionally employ aline of closely spaced photodetector cells that produce signals relatedto the amount of the light reflected from successive swaths of theimage. Most inexpensive consumer scanners have a resolution of 300 dotsper inch (dpi), or a center to center spacing of componentphotodetectors of about 84 μm. Higher quality scanners of the sort foundin most professional imaging equipment and photocopiers have resolutionsof 600 dpi (42 μm), 1200 dpi (21 μm), or better.

[0061] Taking the example of a 300 dpi scanner (84 μm photodetectorspacing), each 250 μm region 12 on the banknote will correspond to about3×3 array of photodetector samples. Naturally, only in rare instanceswill a given region be physically registered with the scanner so thatnine photodetector samples capture the luminance in that region, andnothing else. More commonly, the line art is skewed with respect to thescanner photodetectors, or is longitudinally misaligned (i.e. somephotodetectors image sub-parts of two adjoining regions). However, sincethe scanner oversamples the regions, the luminance of each region canunambiguously be determined.

[0062] In one embodiment, the scanned data from the line art iscollected in a two dimensional array and processed—according to one ofthe techniques disclosed in my prior patents and applications—to detectthe embedded calibration information. The array is then processed toeffect a virtual re-registration of the image data. A software programthen analyzes the statistics of the re-registered data (using thetechniques disclosed in my prior writings) to extract the bits of theembedded data.

[0063] (Again, the reference to my earlier watermark decoding techniquesis exemplary only. Once scanning begins and the data is available inpixel form, it is straightforward to apply any other watermark decodingtechnique to extract a correspondingly-encoded watermark.)

[0064] In a variant embodiment, the scanned data is not assembled in acomplete array prior to the processing. Instead, it is processed inreal-time, as it is generated, in order to detect embedded watermarkdata without delay. (Depending on the parameters of the scanner, it maybe necessary to scan a half-inch or so of the line art image before thestatistics of the resulting data unambiguously indicate the presence ofa watermark.)

[0065] In accordance with another aspect of the invention, varioushardware devices are provided with the capability to recognize embeddedwatermark data in any line art images they process, and to respondaccordingly.

[0066] One example is a color photocopier. Such devices employ a colorscanner to generate sampled (pixel) data corresponding to an input media(e.g. a dollar bill). If watermark data associated with a banknote isdetected, the photocopier can take one or more steps.

[0067] One option is simply to interrupt copying, and display a messagereminding the operator that it is illegal to reproduce currency.

[0068] Another option is to dial a remote service and report theattempted reproduction of a banknote. Photocopiers with dial-outcapabilities are known in the art, (e.g. U.S. Pat. No. 5,305,199) andare readily adapted to this purpose. The remote service can be anindependent service, or can be a government agency.

[0069] Yet another option is to permit the copying, but to insertforensic tracer data in the resultant copy. This tracer data can takevarious forms. Steganographically encoded binary data is one example. Anexample is shown in U.S. Pat. No. 5,568,268. The tracer data canmemorialize the serial number of the machine that made the copy and/orthe date and time the copy was made. To address privacy concerns, suchtracer data is not normally inserted in photocopied output, but is soinserted only when the subject being photocopied is detected as being abanknote. (Such an arrangement is shown in FIG. 7.)

[0070] Desirably, the scan data is analyzed on a line-by-line basis inorder to identify illicit photocopying with a minimum of delay. If abanknote is scanned, one or more lines of scanner output data may beprovided to the photocopier's reprographic unit before the banknotedetection decision has been made. In this case the photocopy will havetwo regions: a first region that is not tracer-marked, and a second,subsequent region in which the tracer data has been inserted.

[0071] Photocopiers with other means to detect not-to-be-copieddocuments are known in the art, and employ various response strategies.Examples are detailed in U.S. Pat. Nos. 5,583,614, 4,723,149, 5,633,952,5,640,467, and 5,424,807.

[0072] Another hardware device that can employ the foregoing principlesis a standalone scanner. A programmed processor (or dedicated hardware)inside the scanner analyzes the data being generated by the device, andresponds accordingly.

[0073] Yet another hardware device that can employ the foregoingprinciples is a printer. A processor inside the device analyzesgraphical image data to be printed, looking for watermarks associatedwith banknotes.

[0074] For both the scanner and printer devices, response strategies caninclude disabling operation, or inserting tracer information. (Suchdevices typically do not have dial-out capabilities.)

[0075] Again, it is desirable to process the scanner or printer data asit becomes available, so as to detect any banknote processing with aminimum of delay. Again, there will be some lag time before a detectiondecision is made. Accordingly, the scanner or printer output will becomprised of two parts, one without the tracer data, and another withthe tracer data.

[0076] Banknotes presently include various visible structures that canbe used as aids in banknote detection (e.g. the seal of the issuingcentral bank, and various geometrical markings). In accordance with afurther aspect of the present invention, a banknote is analyzed by anintegrated system that considers both the visible structures andwatermark-embedded data.

[0077] Visible banknote structures can be sensed using known patternrecognition techniques. Examples of such techniques are disclosed inU.S. Pat. Nos. 5,321,773, 5,390,259, 5,533,144, 5,539,841, 5,583,614,5,633,952, 4,723,149 and 5,424,807 and laid-open foreign application EP766,449.

[0078] In photocopiers (and the like) equipped to detect both visibleand watermarked banknote markings, the detection of either causes one ormore of the above-noted banknote responses to be initiated.

[0079] Again, scanners and printers can be equipped with a similarcapability—analyzing the data for either of these banknote hallmarks. Ifeither is detected, the software (or hardware) responds accordingly.

[0080] Identification of banknotes by watermark data provides animportant advantage over recognition by visible structures—it cannot soeasily be defeated. A banknote can be doctored (e.g. by white-out,scissors, or less crude techniques) to remove/obliterate the visiblestructures. Such a document can then be freely copied on either avisible structure-sensing photocopier or scanner/printer installation.The removed visible structure can then be added in via a secondprinting/photocopying operation. If the printer is not equipped withbanknote-disabling capabilities, image-editing tools can be used toinsert visible structures back into image data sets scanned from suchdoctored bills, and the complete bill freely printed. By additionallyincluding embedded watermark data in the banknote, and sensing same,such ruses will not succeed.

[0081] (A similar ruse is to scan a banknote image on anon-banknote-sensing scanner. The resulting image set can then be editedby conventional image editing tools to remove/obliterate the visiblestructures. Such a data set can then be printed—even on aprinter/photocopier that examines such data for the presence of visiblestructures. Again, the missing visible structures can be inserted by asubsequent printing/photocopying operation.)

[0082] Desirably, the visible structure detector and the watermarkdetector are integrated together as a single hardware and/or softwaretool. This arrangement provides various economies, e.g., in interfacingwith the scanner, manipulating pixel data sets for pattern recognitionand watermark extraction, electronically re-registering the image tofacilitate pattern recognition/watermark extraction, issuing controlsignals (e.g. disabling) signals to the photocopier/scanner, etc.

[0083] (To provide a comprehensive disclosure without unduly lengtheningthe following specification, applicants incorporate by reference thepatent documents cited above.)

[0084] From the foregoing, it will be recognized that embodimentsaccording to the present invention allow line art images to serve assubliminal carriers for binary data. Additionally, existing deterrentsto banknote counterfeiting have been enhanced to prevent commonwork-arounds.

[0085] Having described and illustrated the principles of my inventionwith reference to several illustrative embodiments, it will berecognized that these embodiments are exemplary only and should not betaken as limiting the scope of my invention. Guided by the foregoingteachings, it should be apparent that other watermarking, decoding, andanti-counterfeiting technologies can be substituted for, and/or combinedwith, the elements detailed above to yield similar effects.

[0086] While the invention has been described with reference toembodiments employing regular rectangular arrays of grid points, thoseskilled in the art will recognize that other arrays of points—neitherrectangular nor regular—can alternatively be used.

[0087] While the invention has been described with reference toembodiments that scale the embedded energy in accordance with localimage characteristics, in other embodiments a manually crafted energyprofile can be implemented. That is, a mask defining embedded signalmagnitudes at different parts of the image can be manually devised, andemployed to tailor the change in luminance in each region.

[0088] In view of the many possible embodiments to which the principlesof the invention may be put, it should be recognized that the detailedembodiments are illustrative only and should not be taken as limitingthe scope of my invention. Rather, I claim as my invention all suchembodiments as may come within the scope and spirit of the followingclaims and equivalents thereto.

I claim:
 1. A method of watermarking an image to convey auxiliary data,the image including an element having a contour, characterized in thatthe method includes spatially shifting the location of said contour soas to change the luminosity of at least one image region through whichthe element passes, the change in luminosity conveying the auxiliarydata.
 2. The method of claim 1 in which the auxiliary data comprisesplural bits, and the method includes shifting plural contours.
 3. Themethod of claim 2 in which the image is represented in vector graphicform.
 4. A physical document having a watermarked image formed accordingto the method of claim 1 printed thereon.
 5. A method of watermarking aline art image to convey auxiliary data, the line art image including atleast one line art element having a contour, the method beingcharacterized by spatially shifting the location of said contour.
 6. Themethod of claim 5 in which the spatial shifting of the location of saidcontour effects a change in luminosity of at least one image regionthrough which the element passes, the auxiliary data being detectablethrough the luminosity change.
 7. A physical document having awatermarked line art image formed according to the method of claim 5printed thereon.
 8. The physical document according to claim 7 whereinthe document comprises a banknote.
 9. A method of watermarking an imageto convey auxiliary data, the image including at least one image region,the image region including at least one element, the method beingcharacterized by providing ink droplets in the region while leavingspacing or dimensions of the element otherwise unchanged, the providedink droplets changing the luminance in the region to convey theauxiliary data.
 10. The method of claim 9 wherein the ink droplets areprovided in a vacant part of the region.
 11. The method of claim 9wherein the ink droplets include a diameter of approximately 3 μm. 12.The method of claim 9 wherein the ink droplets are generallyimperceptible.
 13. The method of claim 9 wherein the ink droplets areprovided in an array.
 14. The method of claim 9 wherein the ink dropletsare provided in a random manner.
 15. The method of claim 9 wherein theink droplets are provided as a Gaussian profile.
 16. The method of claim9 wherein relatively dark colored ink droplets are provided to theregion to decrease the luminosity in the region.
 17. The method of claim9 wherein relatively light colored ink droplets are provided to theregion to increase the luminosity in the region.
 18. The method of claim9 wherein the image comprises a banknote.
 19. A method of watermarkingan image to convey auxiliary data, the image including at least oneimage region, the image region including at least one line art element,the method being characterized by providing relatively light inkdroplets to the line art element to form a relatively light void in theline art element, the void changing the luminance in the region toconvey the auxiliary data.